Browsing by Author "Diaz-Droguett, D. E."

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    Chemical and structural analysis related to defects in nanocrystalline Ba1-xSrxTiO3 grown via hydrothermal sol-gel
    (2014) Fuentes, S.; Cespedes, F.; Padilla-Campos, L.; Diaz-Droguett, D. E.
    The chemical and structural properties of Ba1-xSrxTiO3 (BST, x=0-1) nanoparticles synthesised via sol-gel-hydrothermal were analysed. Two types of salts of Ba (BaCl2 and Ba(OH)(2)) and Sr (SrCl2 and Sr(OH)(2)) as starting reactants were used to compare two synthesis methods. Chemical characterisation and oxidation states were obtained using X-ray photoelectron spectroscopy. Structural information was acquired by Raman spectroscopy, and calculations to obtain theoretical Raman spectra associated with the different formed phases of BST were performed for comparison. The results were consistent with the presence of oxygen vacancies in all of the compounds synthesised, but the use of hydroxide salts introduced a minor concentration of oxygen vacancies into the BST compounds. In addition, the presence of oxygen vacancies produced an increase of the intensity of first-order modes of vibration and a minor oxidation state of the Ti atom in the structure. Finally, the oxygen vacancies produced a distortion of the structure, inducing the existence of the non-perovskite phase. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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    Comparative study on the hydrogen storage capacity of crystalline and amorphous nanomaterials of moo3: effect of a catalytic Pd capping
    (2018) Rojas, S.; Roble Albeal, Martín Cristián; Morales-Ferreiro, J. O.; Diaz-Droguett, D. E.
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    Enhanced hydrogen adsorption properties of mesoporous nano-TiO2@SnO2
    (2023) Rajendran, Saravanan; Gnanasekaran, Lalitha; Hoang, Tuan K. A.; Diaz-Droguett, D. E.; Gracia, F.; Gracia-Pinilla, M. A.
    Today, hydrogen as being one of the most advantageous energy supplies for various applications and specifically not with carbon emissions, alluring strategies are made possible for its production and storage. Since the metal oxides (MOs) semiconductors give rise to the development of new technologies related to renewable energy and environmental issues, this work is envisioned to study the hydrogen adsorption properties of TiO2@SnO2 catalyst synthesized via novel approach by integrating sol-gel and thermal decomposition methods. The synthesized TiO2 and its composite have been analyzed by sophisticated instruments to reveal the drastic enhancement of hydrogen adsorption. In this work, hydrogen measurements were accomplished by quartz crystal (QC) microbalance method. The Raman scattering experiment of the composite revealed the lower peak intensity compared with pure TiO2, which demonstrated the surface defects that created oxygen vacancies with reduced oxidation states of the metal centers. Meanwhile, the mesoporous nanostructure of TiO2@SnO2 could be confirmed via High resolution transmission electron (HR-TEM) microscopic analysis. Besides, the intermediate states of the composite were analyzed through Photoluminescence (PL) spectra which demonstrated the delay in recombining charges. The Barrett-Joyner- Halenda (BJH) method illustrated the larger pore size and pore volume with decreased surface area of the composite. The addition of SnO2 into TiO2 has reported 4 times greater the adsorption of pristine TiO2 particles, because of the capacity of SnO2 to hinder pores. Moreover, the titania oxidation states play a predominant role in the procurement of larger H2 adsorption. Also, the Ti4+ and Sn4+ reveal fragile Kubas type of adsorption that facilitated the hydrogen storage.
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    Gas effects on the chemical and structural characeristics of porous MoO3 and MoO3-x grown by vapor condensation in helium and hydrogen
    (2011) Diaz-Droguett, D. E.; Fuenzalida, V. M.
    Porous MoO3 and MoO3-x were grown in vapor-phase under He and H-2 at pressures between 100 and 1200 Pa. MoO3 was vaporized from a tungsten boat heated between 650 and 1250 degrees C. Growth of porous material was promoted at 100 and 200 Pa of the carrier gas. At higher pressures and only in hydrogen under particular preparation conditions, samples rich in porous phase were formed. Samples grown either in helium or in hydrogen have similar aspect under SEM, TEM and XRD, but important differences were detected with thermogravimetry and Raman spectroscopy. The thermogravimetric analysis revealed a hydrated compound in the H-2-grown samples produced at 100 Pa, whereas a non-hydrated compound grew under helium under the same preparation conditions. The Raman spectra revealed some vibrational modes of beta-MoO3 for the He-grown samples at 100 Pa, whereas alpha-MoO3 was only revealed for the H-2-grown samples grown at the same pressure. The effect of the hydrogen pressure on the oxidation state of the porous samples was studied, showing the existence Mo5+ in the porous samples grown at 600 Pa, whereas the samples grown under a lower pressure exhibit the same chemical composition of the source material. The growth kinetics of the porous samples is discussed under different gases as well as the effect of the gas on the chemical and structural characteristics of the samples. (C) 2010 Elsevier B.V. All rights reserved.
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    Growth, characterization and thermo-mechanical analysis of Al/Al2O3 core/shell nanoparticles obtained under H2 atmosphere
    (2020) Diaz-Droguett, D. E.; Ramos-Moore, E.; Roble, M.; Muecklich, Frank
    In this work, growth, characterization and thermo-mechanical behavior of Al/Al2O3 core/shell nanoparticles (NP) is performed. The growth was carried out by gas condensation methods using H-2 as carrier gas at a pressure of 100 Pa and with a temperature of the evaporation source of 1315 degrees C. The prepared NP were characterized by Energy Dispersive X-ray Spectroscopy for chemical information, transmission electron microscopy for morphological study, and electron diffraction patterns for structural information. The prepared Al NP exhibit a thin Al2O3 passivating oxide shell due to the air exposure when removed from the preparation vacuum chamber. Afterwards, the growth of the Al2O3 oxide shell of the Al NP and the thermo-mechanical interaction between this growing oxide shell and its Al core was studied by performing in-situ thermal X-ray diffraction from RT up to 505 degrees C. Coefficients of thermal expansion of both Al core and Al2O3 oxide shell were obtained by means of X-ray strain analysis. It was found that the thickness of the Al2O3 shell increases with temperature and the thermal stress induced in the system increases linearly with temperature. Our results highlight that these NP overcome higher values of fracture toughness compared with Al2O3 NP used in micro-nano composites, thus improving their mechanical properties for nanofluid applications.
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    Influence of crystallographic texture on friction in a-Al2O3/Ti(C,N) ceramic tool coatings
    (2022) Ramos-Moore, E.; Rosenkranz, A.; Diaz-Droguett, D. E.; Espinoza, C.; El Azahri, I.; Muecklich, F.
    This work explores the influence of the crystallographic texture on the resulting frictional response in two ceramic coatings systems containing top alpha-Al2O3 layer with no and fiber texture. The microstructure was analyzed by electron backscatter diffraction, the surface roughness was characterized by white light interferometry, and the micro hardness was measured by standard Vickers testing. The coefficient of friction (COF) was measured using ball-on-disk tribometry under linear reciprocating sliding motion. Our results verified a strong correlation between the crystallographic texture of the top alpha-Al2O3 layer and the resulting COF. This effect can be mainly attributed to the lowest critical shear stress within the basal dislocation slip {0001} < 11-20 >, which facilitates the generation of plastic deformation at the top surface layer. This work highlights the understanding of the complex correlations between microstructure and tribological performance of ceramic coatings.
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    Influence of reactant type on the Sr incorporation grade and structural characteristics of Ba1-xSrxTiO3 (x=0-1) grown by sol-gel-hydrothermal synthesis
    (2013) Fuentes, S.; Chavez, E.; Padilla-Campos, L.; Diaz-Droguett, D. E.
    The influence of barium and strontium starting reactants used in different mole ratios, BaCl2 and Ba(OH)(2), SrCl2 and Sr(OH)2(,) on the chemical and structural properties of Ba1-xSrxTiO3 (x=0-1) (BST) nanoparticles prepared via sol gel-hydrothermal synthesis in an oxygen atmosphere is discussed. The effect of the type of reactant on the relative amount of Sr incorporated in BST compound was also analysed. The synthesised BST nanoparticles showed differences in their structural and chemical characteristics, which were attributed to the presence of Cl- or OH- anions during the synthesis of the compound. The structure, morphology and oxidation state of the samples were studied by X-ray diffraction, transmission electron microscopy and X-ray photoelectron spectroscopy, respectively. In addition, theoretical calculations using cluster models were carried out to understand the possible phases formed of BST, the effect of the Sr incorporation and the possible presence of oxygen vacancies inside the BST structure. (C) 2013 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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    Photosynthesis of H2 and its storage on the Bandgap Engineered Mesoporous (Ni2+/Ni3+)O @ TiO2 heterostructure
    (2020) Raju, Kumar; Rajendran, Saravanan; Hoang, Tuan K. A.; Durgalakshmi, D.; Qin, Jiaqian; Diaz-Droguett, D. E.; Gracia, F.; Gracia-Pinilla, M. A.
    A noble-metal free and surface defect-induced mesoporous mixed valent NiO decorated TiO2 heterostructure with tuned bandgap has been successfully prepared. Its outstanding visible-light driven hydrogen evolution and its excellent H-2 storage ability have been examined and confirmed. The formation of oxygen vacancies by surface defect creates the Ni3+ and Ti3+ on the interface of the heterostructure induce the efficient H-2 evolution, benchmarked by 1200% enhancement in catalytic performance. The underlying chemistries include the near-unity occupancy of e(g) orbital (t(2g)(6) e(g)(1)) of Ni3+ which speeds up the electron transfer and significantly promote the excellent electron-hole separation efficiency, establishes the outstanding overall charge-transfer efficiency and long-term photocatalytic activity in the visible light spectrum. Multiple Ti3+ adsorption centers in the structure attract multiple intact H-2 molecules per each center via a sigma - pi bonding motif - namely the Kubas interaction - which leads to 480% higher H-2 adsorption capability against the performance of the pristine mesoporous TiO2. Not only the significant results, the study also provide an air-stable synthetic method on the basis of low-cost and abundant materials, which are strongly favoured for scaling up production.
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    Structural, optoelectronic and photo-thermoelectric properties of crystalline alloy CuAlxFe1-xO2 delafossite oxide materials
    (2021) Wheatley, R. A.; Roble, M.; Gence, L.; Acuna, C.; Rojas-Aedo, R.; Hidalgo-Rojas, D.; Guzman-De La Cerda, D. E.; Vojkovic, S.; Seifert, B.; Wallentowitz, S.; Volkmann, U. G.; Diaz-Droguett, D. E.
    CuFeO2 and CuAlO2 are attractive candidate materials for solar energy harvesting applications such as photocatalysis and photovoltaics. This work describes the structural, optoelectronic, thermal and electric properties of alloyed CuAlxFe1-xO2 Delafossite material synthesized using solid-state sintering techniques. The alloyed samples of CuAlxFe1-xO2 Delafossite oxide consisted of substitution of Fe for Al ranging from x = 0.01 to x = 0.99. The inclusion of Al in low concentrations affects the crystallization rate during solid-state synthesis, dramatically changing the resultant sample morphologies. The addition of dilute amounts of Al also greatly improves the conductivity of the material to a maximum of 3.18 Scm(-1). The material absorption edge shows strong changes over the alloying range. The more highly conducting samples show a strong photoinduced thermoelectric response to Vis-NIR illumination. (C) 2020 Published by Elsevier B.V.
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    Study of thermally induced phase transformations and microstructural changes in porous MoO3 nanostructures for the design of tribological additives
    (2021) Diaz-Droguett, D. E.; Ramos-Moore, E.; Roble, M.; Muecklich, Frank
    The thermal behavior of MoO3-nanostructures is relevant for the design of tribological additives containing Mo-oxides. We report on thermal behavior of porous MoO3-nanostructures analyzed by X-ray diffraction (XRD), transmission electron microscopy (TEM) and energy dispersive X-ray spectroscopy (EDS). The nanostructures were grown by gas condensation under H-2. Our results revealed 3 stages of phases behavior as a function of temperature. We found dehydration of MoO3-0,33 H2O after 180 degrees C followed by the formation of beta-MoO3 after 230 degrees C, then to alpha-MoO3 after 280 degrees C, and finally formation of suboxide gamma-Mo4O11 at similar to 330 degrees C. The formation of the latter is mainly due to the crystallization and reduction of the porous oxide matrix. Our results highlight that both thermal and chemical processes must be taken into account when designing tribological additives that contain MoO3. (C) 2021 Elsevier B.V. All rights reserved.